ECE 122 Engineering Problem Solving with Java Lecture 16 Class Relationships

1. ECE 122Engineering Problem Solving with JavaLecture 16Class Relationships

2. Outline • Problem: How can I create and store complex objects? • Review of static methods • Consider static variables • What about objects that are stored in other objects • How can I access them? • Moving towards complicated objects

3. The static Modifier •  We declare static methods and variables using the static modifier • It associates the method or variable with the class rather than with an object of that class •  Static methods are sometimes called class methods • Static variables are sometimes called class variables • What happens in memory for static variables and methods?

4. Static Variables • Normally, each object has its own data space • If a variable is declared as static, only one copy of the variable exists private static float price; • Memory space for a static variable is created when the class is first referenced! • All objects instantiated from the class share its static variables •  Changing the value of a static variable in one object changes it for all others • Need to use static variables very carefully

5. Static variables examples public class Particle { public static final int X = 0, Y = 1, Z = 2; .. public int getX () { return position[X]; // use as index } .. } • Static variables are shared across all instances of a class. • They are usually associated with the class itself, rather than a “shared space”

6. Static Class Members • A static method is one that can be invoked through its class name • For example, the methods of the Math class are static: result = Math.sqrt(25) •  Variables can be static as well • Determining if a method or variable should be static is an important design decision • What does static mean to you?

7. Static methods • Can only access static fields • No object variable is passed as an implicit parameter • Can be good for utility methods Math.abs (247); Integer.parseInt (“-37”);

8. Static Methods class Helper { public static int cube (int num) { return num * num * num; } } static method (class method) Because it is declared as static, the method can be invoked as value = Helper.cube(5); Note: the method returns an int… Note: Helper is a class. The cube method is invoked via class name.class method, since NO object needs to be instantiated to use a static method..

9. Static Class Members • The order of the modifiers can be interchanged, but by convention visibility modifiers come first • E.g. public, private • Recall that the main method is static • Static methods cannot reference instance variables • Instance variables don't exist until an object exists • Instance data is unique to each object! • Static method can reference static variables or local variables

10. Static Class Members • Static methods and static variables often work together •  The following example keeps track of how many Slogan objects have been created using a static variable, and makes that information available using a static method •  This is a very popular use of static variables: • Counting the number of objects of a particular class and providing a static method to get (print?) that count out when needed.

11. Class Relationships – Essential Concept! • Classes in a software system can have various types of relationships to each other • Three of the most common relationships: • Dependency: A uses B • Aggregation: A has-a B • Inheritance: A is-a B • Let's discuss dependency and aggregation further • Inheritance is discussed in detail in Chapter 8

12. Dependency •  A dependency exists when one class relies on another in some way, • Usually involves invoking the methods of the other (e.g. System.out.println()…) • We've seen dependencies in many previous examples •  We don't want numerous or complex dependencies among classes • Alternately, some dependence is OK since we need to build a • A good design strikes the right balance

13. Dependency •  Some dependencies occur between objects of the same class • A method of the class may accept an object of the same class as a parameter • For example, the concat method of the String class takes as a parameter another String object str3 = str1.concat(str2); • This drives home the idea that the service is being requested from a particular object • Recall the format of the String methods. They are almost all quite similar: object.method(object)…

14. Dependency • The following example defines a class called Rational to represent a rational number • A rational number is a value that can be represented as the ratio of two integers • Some methods of the Rational class accept another Rational object as a parameter

15. Aggregation • An aggregate is an object that is made up of other objects • Therefore aggregation is a has-a relationship • A car has a chassis • In software, an aggregate object contains references to other objects as instance data • The aggregate object is defined in part by the objects that make it up • Basically the object contains instances of other objects

16. The this Reference • The this reference allows an object to refer to itself • That is, the this reference, used inside a method, refers to the object through which the method is being executed • Suppose the this reference is used in a method called tryMe, which is invoked as follows: obj1.tryMe(); obj2.tryMe(); • In the first invocation, the this reference refers to obj1; in the second it refers to obj2

17. The this reference • The this reference can be used to distinguish the instance variables of a class from corresponding method parameters with the same names • The constructor of the Account class (from Chapter 4) could have been written as follows: public Account (Sring name, long acctNumber, double balance) { this.name = name; this.acctNumber = acctNumber; this.balance = balance; }

18. Summary • Moving toward more complicated objects • Using the same object type in an object • Static variables and methods have important functions • Important to consider the hierarchical use of objects • Building systems increases the focus on problem solving